Dispersed throughout lung airway mucosa are individual and innervated groups of cells with unique morphological, cytochemical and biochemical properties similar to neuroendocrine cells (NE) in the G.I. tract, carotid body, taste bud, thyroid and pituitary. The presence of serotonin, amine precursor uptake and decarboxylation (APUD) enzymes, acetyl cholinesterase, neuron specific enolase and bombesin or at least one or more neuropeptides suggests lung NE cells may have sensory or receptor mechanisms linked with respiratory centers in the CNS. These cells are difficult to locate in adult lung. However, 20 to 30% of all lung tumors are believed to be of NE cell origin possibly in response to environmental influences such as cigarette smoke, ozone, hypoxic conditions, asbestos, halo ethers, and diethylnitrosoamine. It is proposed that these cells play an important role in early lung development since they appear to be the first cells to differentiate and appear to be most numerous during the embryonic and fetal periods. Our objectives are to carry out systematic morphological, cytochemical and biochemical studies of NE cells in the late fetal period, near birth. In order to do so, new systems of analysis need to be developed.